This invention relates to highway safety devices and more particularly to such devices for decelerating uncontrolled or improperly controlled vehicles as they approach immovable obstructions such as bridge abutments or as they leave the highway at a point of particular danger.
As is well known, the matter of reducing the highway fatality and injury rate has been the subject of increasing attention in recent years. The matter of highway safety includes a number of factors such as the quality of the vehicle, the training, skill and experience of the driver, the highway system itself and the establishment and enforcement of intelligent safety laws. Limited progress is being made in all of these areas.
Studies have demonstrated that more than a third of fatal accidents involve only one vehicle. In the usual case the driver loses control of the vehicle which then leaves the road and often strikes a fixed obstruction unavoidably in position adjacent to the highway and which may be a part of the highway system itself such as a bridge abutment. In such a case the occupants of the vehicle can be protected only by controlling the rate of vehicle deceleration to reduce the risk of injury caused by the "second collision" which occurs when the occupants strike the interior of the vehicle.
Since the impact velocity of the vehicle is beyond control and its final velocity is zero, the risk of injury can be reduced only by increasing the distance through which the vehicle and is occupants are decelerated.
Proposals to incorporate crushable structure in the vehicle for this purpose have been, for the most part, unsuccessful. Because of practical limits in vehicle size the crush distance obtainable through the most sophisticated auto design is on the order of two feet and cannot exceed four or five feet in a practical automobile that can be built for a price acceptable to the purchasing public. Obviously a passenger car incorporating even two or three feet of lateral crush distance making the overall width of the vehicle 10 feet or more is not feasible. Similarly, it is not feasible to increase significantly the crush distance of the fixed obstructions which necessarily possess a high degree of rigidity.
Accordingly, it has been proposed to introduce sacrificial energy absorbing devices in the path of the vehicle adjacent to heavy fixed obstructions. Despite the need for such devices and their obvious advantages, insofar as presently known none have been accepted in any significant number either because of lack of efficiency, prohibitive cost or because in some cases they increase the hazard or become hazardous themselves. Examples of such prior proposals may be found in U.S. Pats. 2,088,087; 2,375,443; 3,141,655; 3,288,440, and 3,292,909.
Among the deficiencies of most, if not all, of these prior proposals in their tendency to substitute one uncontrolled motion for another, i.e., to deflect the vehicle into the path of an oncoming vehicle; to improse an overturning moment on the vehicle, to create a secondary hazard, i.e., where the energy absorbing devices themselves may be projected into the path of outer vehicles, or to subject the vehicle and its occupants to severe deceleration forces.
The familiar guard rails of various types, median dividers, fences, and walls are "tangent deflection" devices intended to deflect any vehicle which strikes them tangentially. For the most part they are intended simply to change the direction of the vehicle without appreciably reducing its velocity or momentum. Since the tangent deflection devices are usually anchored and usually have greater rigidity than vehicles, they constitute in themselves further obstructions along the highway, especially when the ends of "tangent deflection" barriers are exposed to vehicle impingement. Then when they are struck by a vehicle travelling in a path at right angles or at a high angle to the barrier, the results are often as severe as when a vehicle strikes a bridge abutment or other fixed obstruction. This is because the "tangent deflection" barrier is, in these cases, being misused to perform the function of a "head-on" type of barrier. Most, if not all, of the prior barriers are by design intended to arrest or deflect the movement of the vehicle within a fixed, usually very short, distance regardless of the speed and weight of the vehicle. The result generally is the imposition of catastrophically high forces on the vehicle and the occupants.